Recent advances in technology have given rise to communications electronics that are faster, consume less power and are less expensive as compared to earlier generations. This in turn has caused rapid growth in the global communications market, which includes both fixed and mobile segments. This rapid growth has manifested itself through increasing numbers of users of communications technologies, and the increasing services and bandwidth available to users. This growth is expected to continue for many years to come.
Current technologies for multi-user communication systems include code division multiple access (CDMA) and time division multiple access (TDMA), both of which are widely implemented in mobile communications. TDMA is used in the United States (IS-136) and Europe (GSM) as a digital wireless technology. CDMA (IS-95) has been implemented in digital wireless systems in the past few years and exhibits certain improved performance characteristics. CDMA accordingly appears poised to overtake TDMA and become the preferred technology for the third generation mobile communications systems, which seek to provide high-speed data services in addition to providing high-quality voice services.
CDMA and TDMA have different performance characteristics in several areas. CDMA works by coding message bits into code sequences, which in turn are modulated for transmission over a wireless channel. In contrast to TDMA, the coding allows correction of some transmission errors due to noise in the channel, at least when there is less than full occupancy of the communication channel.
In a widely used mobile cellular implementation of CDMA, up to 64 (or 256) signals are transmitted in parallel from a base station to mobile units. In realistic noise environments, this number is limited by the peak power that can be transmitted by law or other considerations. There is accordingly a necessary balance between the transmitted signal power of the composite CDMA signal and the number of parallel CDMA active users supported. Although a higher transmitted signal power will usually result in a better coverage and signal reception at the receivers, this will also result in higher noise in neighboring cells. A performance indicator for mobile communication systems is the peak-to-average power (PAP) magnitude of the composite CDMA signals. High PAP has always been an inherent problem of CDMA systems. Pulse shaping and complex modulation techniques such as continuous phase modulation techniques have been developed to alleviate negative effects of high PAP.
Despite the aforementioned techniques and because of the aforementioned techniques, problems persist. The problems may manifest themselves, for example, through the introduction in CDMA systems of data channels devoted to pulse shaping and complex modulation techniques. Devoting channels to this purpose may lower the overall bandwidth of the system. Another consequence is that CDMA systems may require more expensive electronics, such as linear power amplifiers with high dynamic range, to handle signals with high PAP or with many data channels. This can be particularly problematic for mobile communication units where the cost per unit is sensitive.
Accordingly, there is a need for a new system and method for leveraging the advantages of CDMA that can increase the performance of CDMA to allow its operation in a third generation environment of high-data rates. There is a further need for a system which alleviates problems associated with high PAP. There is still a further need for such improvements in CDMA performance to be capable of low-cost implementation in hardware, software or firmware within existing CDMA systems to realize performance improvements. There is still a further need for systems which reduce PAP thus eliminating or reducing the need for signal shaping data channels or expensive electronics, such as linear power amplifiers with high dynamic range.
Alternatively, there is also a need for developing codes with better tolerance to non-linear distortions when transmitted as a high PAP composite signal.